1. Field of the Invention
The present invention relates to a droplet-jetting device such as an ink-jet head of an ink-jet printer.
2. Description of the Related Art
An apparatus has been hitherto suggested, in which a piezoelectric droplet-jetting device is utilized for a print head. This device is constructed such that the volume of a liquid chamber is changed by the dimensional displacement of a piezoelectric actuator, and thus the liquid (ink) contained in the liquid chamber is jetted from a nozzle during the decrease of the volume, while the ink is introduced into the liquid chamber during the increase of the volume. A large number of the droplet-jetting devices as described above are arranged closely to one another, and the ink is jetted from the droplet-jetting device disposed at a predetermined position. Accordingly, a desired letter or an image is formed.
For example, FIG. 28 shows an ink-jet print head which utilizes the conventional piezoelectric droplet-jetting device. FIG. 28 shows a magnified sectional view illustrating the conventional piezoelectric ink-jet head. The piezoelectric ink-jet head comprises nozzles 215 which are open to the outside, pressure chambers 216 which supply the ink to the nozzles 215, a common ink chamber 212a which distributes the ink from an unillustrated ink supply source to the plurality of pressure chambers 216 via ink supply holes 218, 216b and throttle sections 216d, and a piezoelectric actuator 220 provided with pressure-generating sections 228 which apply the pressure to jet the ink to the pressure chambers 216.
The pressure-generating section 228 is a portion of the piezoelectric actuator 220 at which a piezoelectric sheet 222 of the piezoelectric actuator 220 is interposed between a driving electrode 224 and a common electrode 225. The pressure-generating section 228 is subjected to the polarization treatment in a direction directed from the driving electrode 224 to the common electrode 225. When an electric field, which matches the direction in which the polarization treatment is applied, is applied between the driving electrode 224 and the common electrode 225, the pressure-generating section 228 causes the elongation displacement in the thickness direction of the piezoelectric actuator 220. As a result of the displacement, the volume of the pressure chamber 216 is decreased, and the ink contained in the pressure chamber 216 is extruded. Accordingly, ink droplets are jetted from the nozzle 215 which is communicated with the pressure chamber 216.
In order to jet the ink droplets having necessary jetting velocities and volumes more efficiently, i.e., at a lower voltage, the pressure-generating section 228 has been arranged in a region approximately ranging over the entire pressure chamber 216.
However, the conventional piezoelectric ink-jet print head as described above has involved the following problems, because the pressure-generating section has been arranged in the region approximately ranging over the entire pressure chamber. That is, the electrostatic capacity, which is proportional to the area of the pressure-generating section, is increased. The energy efficiency is unsatisfactory. The power source system, which is used to drive the ink-jet print head, suffers from the increase in cost.
The piezoelectric ink-jet print head as described above is suitable for the so-called “push-eject” in which the ink droplets are jetted by decreasing the volume of the pressure chamber when the driving voltage is applied. However, when such a method is used, a problem arises such that the supply of the ink is not performed in time, and it is impossible to increase the driving frequency so much. Further, when such a method is used, a problem arises such that the volume of the ink droplet cannot be increased so much as well.
Therefore, it is intended to perform the so-called “pull-eject” as a method for increasing the driving frequency and increasing the volume of the droplet, in which the volume of the pressure chamber is firstly increased, and then the volume of the pressure chamber is restored to the original volume at the timing at which the pressure in the pressure chamber is changed from the negative to the positive. In this case, it is necessary to use such a method that the volume of the pressure chamber is always decreased by always applying a voltage, and the voltage application is shut off only when the printing operation is performed. Therefore, the energy efficiency has been extremely unsatisfactory.
In such a method, it is also conceived that a reverse electric field is applied in order to increase the volume of the ink chamber. However, if such a procedure is adopted, only a low electric field, which causes no polarization reversal, can be applied. It is impossible to jet any sufficient amount of ink droplets.